1. Field of the Invention
The present invention relates to an ink jet recording head, and more particularly to an ink jet recording head of a thermal ink jet type.
2. Description of the Prior Arts
There has been known an ink jet recording head of a thermal ink jet type as an ink jet recording apparatus wherein an ink droplet is ejected onto a recording sheet according to an image signal for recording an image, a character and the like.
The ink jet recording apparatus of such a thermal ink jet type has, for example, an ink jet recording head; 70 having a construction shown in FIG. 7. This ink jet recording head 70 is provided with a heating substrate 52 and a flowing path substrate 50 formed on the heating substrate 52. The heating substrate 52 has electricity/heat converting elements 60, each of which is arranged at a position corresponding to each ink flowing path 58 described later and has the same size. An electric pulse is applied to the electricity/heat converting element 60 via a device not shown or wiring according to an image signal.
Provided at the flowing path substrate 50 are an ink supplying opening 56 that introduces ink from an ink tank not shown, an ink chamber 54 for temporarily keeping ink introduced from the ink supplying opening 56 and plural ink flowing paths 58, which is ends on one side are open to the ink chamber 54 and ends on the other side form an ejecting opening 59.
The ink chamber 54 and the ink flowing path 58 are filled with ink upon recording an image. When the electric pulse is applied to the electricity/heat converting element, i.e. thermal element, 60 from the heating substrate 52 according to the image signal, the electricity/heat converting element 60 generates heat to form a bubble at the heating portion. This bubble brings a pressure to ink in the ink flowing path to eject ink from the ejecting opening 59, whereby the ejected ink is adhered onto a recording sheet arranged so as to oppose to the ejecting opening 59, thereby recording an image.
Generally, a high resolution of 600 dpi or more or an ejection of an extremely minute ink droplet of 10 pl or less are required for recording an image of a high quality. In the case of recording an image of a high quality by using the ink jet recording head of a thermal ink jet type having the above-mentioned construction, a period for one scan increases since an area where an ink droplet is adhered by one ejection is small. Further, a relative moving speed of the head in the slow-scanning direction becomes slow, resulting in considerable slowdown of the printing speed.
Moreover, the smaller the size of the ejected ink droplet is made for obtaining an image of higher quality, the less the ink amount adhered onto the recording sheet for one scan becomes, whereby the density of the image becomes low. Therefore, it is required that a slow-scan is repeated in plural times for ejecting ink droplets in plural times at the same position on a recording sheet to adjust the density to a desired one (multi-scan). This brings a slower printing speed.
Accordingly, printing is conventionally performed by changing a dot number between normal printing and high-speed printing. As one of such methods, a method has been known wherein an image is formed by all dots as shown in FIG. 8A in normal printing while an image is formed in high-speed printing by dots fewer than the dots in number in normal printing as shown in FIG. 8B, i.e., an image is formed by thinning-out printing, thereby enhancing a relative printing speed in the fast-scanning direction and slow-scanning direction to perform high-speed printing.
Japanese Published Unexamined Patent Application No. Hei 8-332727 discloses an ink jet recording head having large and small heaters 62 and 64 arranged laterally in an ink flowing path 58 as shown in FIG. 9A, or arranged longitudinally in the ink flowing path 58 as shown in FIG. 9B, wherein the small heater 62 is turned on in the normal printing to eject small ink droplets, while only the large heater 64 or both of the small and large heaters 62 and 64 are turned on in high-speed printing to eject large ink droplets.
Generally, ejectable ink droplets having a smaller size is preferable for recording an image with high resolution, while ejectable ink droplets having a larger size is preferable for performing thinning-out printing to print at a high speed. Therefore, it is required to have a construction such that the size of ink droplets which can be ejected from a single ink jet recording head can be greatly switched over.
For example, a volume of an ink droplet required for forming dots on a sheet with a resolution of 800 dpi without a space is approximately 7 pl to 15 pl per one droplet. In order to obtain an image with higher resolution than 800 dpi for rendering graininess unnoticeable, ink droplet having smaller volume is required. On the other hand, in the case where dots are thinned out every other one dot for high-speed printing, a volume of an ink droplet required for filling the space between each dot with a resolution of 400 dpi is approximately 20 to 50 pl per droplet.
Specifically, it is desired that a ratio of a dot diameter of the minimum ink droplet that can be ejected from a single ink jet recording head and a dot diameter of the maximum ink droplet has a wide range of at least approximately 1:3 to 1:7.
However, it is difficult to greatly change the size of an ink droplet in the ink jet recording head having the conventional construction.
For example, in the case where two heaters 62 and 64 are arranged laterally in the ink flowing path 58 as shown in FIG. 9A, the volume of an ejectable ink droplet is limited by the size of the heater (i.e., amount of generated heat) and the width of the flowing path.
Specifically, if the width of the flowing path is enlarged, the size of the heater that can be arranged can be made large, so that an ink droplet having greater volume can be ejected to increase the dot diameter that can be formed. However, since the resolution at the ejecting opening becomes low, that brings a wide space between each dot formed by ejecting ink droplets of a small volume, thereby unpreferable. Accordingly, the limit is the volume ratio of approximately 1:2 of a volume of the minimum ink droplet that can be ejected from the ink jet recording head and a volume of the maximum ink droplet in the case of laterally arranging the heating element.
Further, in the case where two heaters 62 and 64 are longitudinally arranged in the ink flowing path 58 shown in FIG. 9B, the size of the heating element can be enlarged without widening the width of the flowing path compared to the case where the heating element is laterally arranged. However, a signal electrode for selecting two heating elements and an electrode for applying a voltage need to be wired for every flowing path. Since the width of the flowing path for such electrodes are required, a heating element having a sufficient size cannot be arranged. Therefore, the limit is the volume ratio of approximately 1:2.5 of a volume of the minimum ink droplet that can be ejected from the ink jet recording head and a volume of the maximum ink droplet.
If the width of the flowing path is enlarged for ejecting ink droplets having a larger volume, a low resolution at the ejecting opening is inevitable. Although a slight improvement is made compared to the case where the heating element is laterally arranged, it is impossible to greatly increase the ratio of the dot diameter of the minimum ink droplet to the dot diameter of the maximum ink droplet.
Moreover, the length of the flowing path becomes long in the longitudinal arrangement compared to the lateral arrangement, thereby enlarging a fluid resistance from the ink chamber to the orifice. Therefore, this longitudinal arrangement has a problem that the resupply of the ink jet becomes slow after ejecting ink droplets, whereby high-speed printing cannot be performed.
Specifically, the conventional ink jet recording head has a disadvantage that, when the resolution in the normal printing is increased so as priorly to obtain an image of high quality, a large dot cannot be formed in high-speed printing, thereby making a space between each dot formed onto a recording sheet. Therefore, an image has a lower density than the desired density. Conversely, the resolution in normal printing is decreased when high-speed printing takes priority, thereby degrading image quality.
The present invention has been made in view of the above circumstances and provides an ink jet recording head in which an ink droplet having a small volume that can realize a desired image quality can be ejected onto a recording sheet without making a space between dot""s in the normal printing, and further, in which an ink droplet having a large volume that can afford a sufficient density can be ejected even when a thinning-out printing is performed in high-speed printing.
The present invention provides a construction such that an ink jet recording head has a first ink flowing path that ejects ink supplied from one side thereof from an ejecting opening at the other side thereof as an ink droplet of a constant amount and a second ink flowing path that can eject ink supplied from one side thereof from an ejecting opening at the other side thereof as an ink droplet of an amount more than the constant amount as well as that can adjust the amount of the ink droplet to be ejected, wherein sets of ink flowing paths including the first ink flowing path and the second ink flowing path are alternately arranged.
Set of ink flowing paths having the first ink flowing path, the ejecting amount of which is constant, and the second ink flowing path, the ejecting amount of which is variable, are alternately arranged, whereby an ink droplet is ejected at a constant interval either in the case where small ink droplets are ejected from all ink flowing paths and in the case where only the second ink flowing path is used for ejecting large ink droplets from the second ink flowing path. Therefore, space between dots formed onto a recording sheet does not vary, so that the dots can be arranged without space.
Accordingly, an image can be formed with a resolution corresponding to pitches of the first and second ink flowing paths in the case of ejecting small ink droplets from the first and second ink flowing paths, while an image can be formed with a resolution corresponding to a pitch of the second ink flowing path in the case of ejecting large ink droplets from the second ink flowing path.
Further, the present invention also provides an ink jet print head on which an amount of the minimum ink droplet ejected from the second ink flowing path becomes the same as an amount of an ink droplet ejected from the first ink flowing path, no variation occurs in the diameters of dots recorded onto a recorded sheet formed by the ink droplet ejected from the first ink flowing path and the minimum ink droplet ejected from the second ink flowing path. Further, an image can be formed with a resolution corresponding to pitches of the first and second ink flowing paths.
In this case, the first ink flowing path has a constant ejecting amount, so that it is sufficient to have a flowing width required for ejecting ink droplets in a smaller amount. Therefore, the flowing width of the first ink flowing path in the fast-scanning direction can be formed narrow compared to the case where the ejecting amount from all of the ink flowing paths is made variable. Accordingly, the flowing width of the second ink flowing path that ejects ink droplets in the same amount as the ink droplets ejected from the first ink flowing path as the minimum size ink droplet can be widened compared to the case where the ejecting amount from all of the ink flowing paths is made variable, thereby being capable of arranging a larger heating element compared to the conventional ink jet recording head having a construction that the ejecting amount from all of the ink flowing paths is made variable.
By this construction, it may be possible to establish the volume ratio of 1:3 or more of the volume of the minimum ink droplet that can be ejected from the ink jet recording head to the volume of the maximum ink droplet that can be ejected from the ink jet recording head. Therefore, the problem does not occur that, when the resolution in the normal printing is increased so as pirorly to obtain an image of high quality, the space between dots formed onto a recording sheet is enlarged in high-speed printing to obtain an image having a lower density than the desired density, and conversely that, when high-speed printing takes priority, the resolution in the normal printing is decreased to degrade image quality. Accordingly, an ink jet recording head can be obtained wherein a satisfactory image can be obtained in high-quality printing as well as an image having satisfactory density can be obtained in high-speed printing.
Moreover, the ink jet recording head may have the construction, disclosed in another aspect of the present invention, that ink is ejected by an electricity/heat converting element that converts electricity into heat. By using the electricity/heat converting element, the amount of ink droplet can be controlled with high precision by a relatively simple construction.
Further, a heating element or the like in which the amount of generating heat increases according to the value of the applied voltage can be used for ejecting ink droplets from the second ink flowing path. This heating element may be made of plural electricity/heat converting elements as disclosed in another aspect of the present invention.
In this case, when the second ink flowing path ejects the minimum size ink droplet, one or predetermined numbers of electricity/heat converting elements may be controlled to be driven, while electricity/heat converting elements more in number than the one or predetermined numbers of electricity/heat converting elements may be controlled to be driven when the second ink flowing path ejects the ink droplets larger than the minimum size ink droplet.
Moreover, the volume of the ejected ink droplet varies according to the amount of generating heat of the driven electricity/heat converting element, so that it is possible to selectively eject ink droplets of volumes of two kinds or more by performing a close control such that the number of the driven electricity/heat converting elements is changed two, three, four.
Further, the ink jet recording head of the present invention may have the construction that the electricity/heat converting element for the minimum size is driven when ejecting the minimum size ink droplet, while the electricity/heat converting element for a large ink droplet is driven when ejecting the large ink droplet.
Moreover, the electricity/heat converting element that is driven when ink droplets of the least amount are ejected from the second ink flowing path, i.e., the one or predetermined numbers of the electricity/heat converting elements, may be arranged at the nearest position from a projecting opening of the second ink flowing path, ink droplets in the same amount as ink droplets ejected from the first ink flowing path can be ejected with high precision from the second ink flowing path having the flowing width larger than that of the first ink flowing path.
Additionally, when a dot having a large diameter is recorded by the ink droplet ejected from the second ink flowing path, a relative moving speed in a slow-scanning direction may be increased according to a ratio of a diameter that is made large compared to a relative moving speed in the slow-scanning direction when a dot having a small diameter is recorded.
Specifically, an area covered by dots formed on the recording sheet by an ejected ink droplet in high-speed printing may be enlarged according to the ratio of the increasing diameter with respect to an area covered by dots formed on a recording sheet by ejected ink droplets in high quality image printing.
Therefore, at least either one of the moving speed in the slow-scanning direction of the ink jet recording head or the feeding speed of the recording sheet may be controlled such that the relative moving speed in the slow-scanning direction of the ink jet recording head in high-speed printing is increased according to the ratio of the diameter that is made large compared to the relative moving speed in the slow-scanning direction of the ink jet recording head in high quality image printing.
By this, dots formed on a recording sheet by the ejected ink droplets may be arranged with a satisfactory space therebetween, not excessively overlaying with each other and not having an excessive space therebetween.
Additionally, when a dot having a large diameter is recorded by the ink droplet ejected from the second ink flowing path, an ejecting frequency is decreased according to a ratio of a diameter that is made large compared to an ejecting frequency when a dot having a small diameter is recorded.
Specifically, an area covered by dots formed on a recording sheet by an ejected ink droplet in high-speed printing is enlarged according to the ratio of the increasing diameter with respect to an area covered by dots formed on a recording sheet by ejected ink droplets in high quality image printing. Therefore, the number of dots formed on the recording sheet decreases for the enlarged area.
Therefore, when a dot having a large diameter is recorded by a plenty of ink droplets ejected from the second ink flowing path, the ejecting frequency is decreased according to the ratio of the diameter that is made large, whereby dots formed on a recording sheet by the ejected ink droplets are arranged with a satisfactory space therebetween, not excessively overlaying with each other and not having an excessive space therebetween.
In this case, the relative moving speed of the ink jet recording head in the slow-scanning direction dose not change. However, dots formed on a recording sheet by the ejected ink droplets are arranged with a satisfactory space therebetween, not excessively overlaying with each other and not having an excessive space therebetween, with the result that there is no disadvantage such as a thin density. Therefore, a sufficient density can be obtained by one ejection. The printing operation completes faster since the compensation processing after that is not required.
When a magnification of a maximum resolution to a minimum resolution by the ink droplet ejected from the second ink flowing path is rendered to be n, first ink flowing paths in the number of nxe2x88x921 and a single second flowing path may be arranged in the set of the ink flowing paths as described in another aspect of the present invention.